Knowledge of the characteristics of visual arrays that "guide" attention to particular stimuli within the array is critical for rapidly establishing effective observing behavior and stimulus control. Much of our past research has emphasized the role of the structure of visual arrays in facilitating detection of relevant stimulus relations as a "front-end" manipulation. We believe a delineation of the structure of visual stimulus information is critical for specifying the constraints on (a) the acquisition of rule-based behaviors, and (b) the detection of stimulus similarity and difference by individuals with and without mental retardation. To this end, we have had much success in "guiding" attentional processes across a range of visually-based experimental tasks (e.g., oddity and matching-to-sample). The current proposal is designed to further our understanding of structural characteristics of visual arrays that function to "guide" attention to the critical stimulus (i.e., the target or S+) in a visual presentation. In addition, we are interested in determining characteristics of visual arrays that "capture" the initial observing response of the individual. An understanding of these factors then could be utilized to increase the probability that the task-relevant stimulus will be selected, resulting in more efficient establishment of control by the intended stimuli. The results of the proposed experiments will demonstrate the application of basic research on pre-attentional processing to educationally-relevant contexts (e.g., match-to-sample training and visual processing in naturalistic scenes). Thus, an important goal of the project is to inform the design of more efficient training procedures and computer- assisted learning programs for individuals with mental retardation.